Ab initio calculations
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The aromatic stabilization of cyclic phosphenium cations (general type C2N2P+) was studied by experimental methods (synthesis, multinuclear NMR, single crystal X-ray crystallography) and quantum chemical calculations (ab initio methods). The structures of the 1,3,2-diazaphosphole derivatives [(tBuN-CH=CH-NtBu)P+]Cl- (1), (tBuN-CH2-CH2-NtBu)P-Cl (2), [(tBuN-CH=CH-NtBu)P]+PF6- (3) and [(tBuN-CH2-CH2-NtBu)]P+PF6- (4) were examined by single crystal X-ray diffraction. The chloro phosphane [(tBuN-CH=CH-NtBu)P]+Cl- (1) has an ionic P-Cl bond and contains an aromatically stabilized phosphenium cation [shortest distance P···Cl = 275.9(2) pm], while the CC-saturated compound (tBuN-CH2-CH2-NtBu)P-Cl (2) is covalent. The two chloro-phosphanes 1 and 2 differ sharply in their volatility and solubility in organic solvents. Compound 2 is soluble in hydrocarbons and sublimes readily at 90 °C/0.1 Torr but 1 is insoluble in hexanes and not volatile below 180 °C/0.1 Torr. The degree of aromatic stabilization in the phosphenium cation 1 was investigated by ab initio methods. For the model cations [RN-CH2-CH2-NR]P+ and [(RN-CH=CH-NR)P]+, thermochemical stabilization energies of 25.8 kcal · mol-1 (R = H) and 28.1 kcal · mol-1 (R = Me) were obtained from isodesmic hydrogenation reactions at the RHF/MP2/6-31G*//RHF/6-31G* level.
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